In a SAS domain, discovery I/O operations must compete with all other I/O operations. A problem can arise when the I/O traffic level is so high that it crowds out the discovery traffic, which can in turn make it difficult for discovery to complete in a timely manner. In some cases, discovery may not complete at all as a result of the congestion. Even in the best case, discovery in large topologies can take more time than is desired. There is, therefore, a continuing need for selectively blocking I/O traffic in a SAS domain. More particularly, there is a need for blocking and unblocking I/O traffic for selected physical or logical devices connected to a common SAS expander or initiator.